Vehicles using a high-voltage battery pack generally include a main contactor to switch battery power to the power electronic components. There is generally some capacitance associated with the power electronics circuitry. This capacitance can create large inrush currents when closing the main contactor. These large inrush currents can create an arc across the contacts causing damage to or reducing the operating life of the contacts and creating sparks. To eliminate the large inrush currents and to protect the main contactor, a pre-charge circuit is often used in parallel with the main contactor.
A typical pre-charge circuit may consist of a contactor in series with a resistor connected across the main contactor. During power-up of the vehicle, the pre-charge contactor is closed and current flows through the pre-charge contactor and resistor. The resistor is selected to limit the amount of current that flows. When the bus has reached a desired voltage or is within a desired range, the main contactor may be switched on and the pre-charge contactor may be switched off. Although these pre-charge circuits may reduce the voltage across the main contractor during switch-on, there may still be some small voltage across the main contactor that may damage the contactor over time.
A typical pre-charge event is ended by measuring the voltage difference between the battery output and the main contactor output using a voltage divider circuit. A high-precision voltage divider circuit is required to measure a near zero voltage for a high voltage over typical automotive temperature ranges. In addition, this type of circuit may be required on the battery output and the main contactor output which adds additional cost to the vehicle.